To provide electrical conductivity between layers in a semiconductor device, an interconnect or via may be formed through an ILD. The via is then lined with a barrier and filled with an electrically conductive material such as Cu to provide electrical conductivity between the layers. As semiconductor components continue to be scaled down and placed closer together, ILDs have become thinner, requiring lower-k dielectric materials, e.g. ULK dielectrics, to be used for improved electrical performance. For electromagnetic (EM) improvement, a Co cap is formed over a Cu via, e.g. by selectively depositing Co on top of the Cu. Selectivity is determined by the amount of Co thickness on top of the Cu divided by the amount of Co thickness on top of the ULK ILD. As Co is deposited on Cu, carbon (C) remains on the surface, which reduces the deposition rate over time. Moreover, too much Co deposited on the ULK ILD surface will cause time-dependent dielectric breakdown (TDDB) failure. Consequently, ammonia (NH3) plasma is used to recover the deposition rate by removing impurities, e.g., C ring groups, from the Co surface. However, with each NH3 plasma treatment, selectivity is reduced between the Cu and ULK ILD surfaces, and damage to the ULK is increased.
A need therefore exists for methodology enabling selective Co capping of a Cu interconnect while protecting the ULK ILD surface.